1. Field of the Invention
The present invention generally relates to the prophylactic detection of impending chemical volatility, and in particular to use of imaging techniques to detect the presence of volatile organic compounds outside a containment system.
2. Background Description
Petroleum refineries and organic chemical manufacturers periodically inspect leaks of volatile organic compounds (VOC) from equipment components such as valves, pumps, compressors, flanges, connectors, pump seals, etc. as described in L. Zhou, and Y. Zeng, “Automatic alignment of infrared video frames for equipment leak detection,” Analytica Chimica Acta, Elsevier, v. 584/1, pp. 223-227, 2007. Common practice for inspection is to utilize a portable flame ionization detector (FID) sniffing the seal around the components for possible leaks, as indicated by the U.S. Environmental Protection Agency in “Protocol for Equipment Leak Emission Estimates,” EPA-453/R-95-017, November 1995. A single facility typically has hundreds of thousands of such components.
FIDs are broadly used for detection of leakage of volatile organic compounds (VOC) in various equipment installed at oil refineries and factories of organic chemicals. For example, U.S. Pat. No. 5,445,795 filed on Nov. 17, 1993 describes “Volatile organic compound sensing devices” used by the United States Army. Another invention by the same inventor, U.S. Patent Application No. 2005/286927, describes a “Volatile organic compound detector.” However, FID based monitoring approaches turns out to be tedious work with high labor costs even if the tests are carried out on as limited a frequency as quarterly.
Several optical imaging based methods are proposed in the literature for VOC leak detection as a cost-effective alternative, as described in ENVIRON, 2004: “Development of Emissions Factors and/or Correlation Equations for Gas Leak Detection, and the Development of an EPA Protocol for the Use of a Gas-imaging Device as an Alternative or Supplement to Current Leak Detection and Evaluation Methods,” Final Rep. Texas Council on Env. Tech. and the Texas Comm. on Env. Quality, October, 2004, and M. Lev-On, H. Taback, D. Epperson, J. Siegell, L. Gilmer, and K. Ritterf, “Methods for quantification of mass emissions from leaking process equipment when using optical imaging for leak detection,” Environmental Progress, Wiley, v. 25/1, pp. 49-55, 2006. In these approaches, infra-red (IR) cameras operating at a predetermined wavelength band with strong VOC absorptions are used for leak detection.
In other contexts it has been shown that fast Fourier transforms can be used to detect the peaks inside a frequency domain. For example, in the video-based fire detection system developed by Fastcom, temporal fast Fourier transforms were computed for the boundary pixels of objects, as described in R. T. Collins, A. J. Lipton, T. Kanade, H. Fujiyoshi, D. Duggins, Y. Tsin, D. Tolliver, N. Enomoto, O. Hasegawa, P. Burt, and L. Wixson, “A System for Video Surveillance and Monitoring: VSAM Final Report,” Tech. report CMU-RI-TR-00-12, Carnegie Mellon University, 2000. In a similar system developed by Liu and Ahuja, shapes of fire in the video were also represented within frequency domain, as described in B. U. Toreyin, A. E. Cetin, A. Aksay, and M. B. Akhan, “Moving Object Detection in Wavelet Compressed Video,” Elsevier, Signal Processing: Image Communication, EURASIP, vol. 20, pp. 255-264, 2005 (hereafter “Signal Processing 2005”). Since Fourier transforms don't contain temporal information, these transforms should be performed inside previously established time frames. Within these time frames, length of the time frame plays a vital role. If length of the time frame is too long, not too many peaks may be obtained in fast Fourier transform data. If length of the time frame is not long enough, then no peaks may be obtained in fast Fourier transform data. However, VOC plumes exhibit variations over time that are random rather than according to a purely sinusoidal frequency. This means that Fourier domain methods are difficult to apply to VOC plume detection.
Volatile organic compounds are typically stored in containers and piped through systems using valves, connectors, pump joints, and similar equipment. While this equipment is designed so that the VOC remains contained within the system, there is potential for leakage at these valves, connectors, pump joints and the like. To detect leakage a detector is positioned in the vicinity of such equipment. At these locations, the detector makes separate measurements at each piece of equipment to determine whether or not there is a VOC plume. In the prior art gas leakage in the form of VOC plumes is detected using methods like gas chromatography, as described in Japanese Patent No. JP2006194776 for “Gas Chromotograph System and VOC Measuring Apparatus Using it” to Y. Tarihi, or oxidation as described in Patent No. WO2006087683 for “Breath Test for Total Organic Carbon”. However, these processes cause loss of time, effort and money at places, such as oil refineries, where there are many pieces of equipment that are likely to incur leakage.
Therefore there is a need for a VOC plume detection technology that is not constrained by the foregoing limitations of the prior art.